The present invention relates to a thermal treatment apparatus for producing a carbon material which is employed as a filler; i.e., which is added, as a composite material, to resin in order to improve the physical properties of the resin, such as electrical conductivity and heat conductivity, or which is employed in a variety of batteries such as a lithium-ion battery which has recently become of interest; and to a thermal treatment method for producing the graphite carbon material.
In recent years, portable electronic apparatuses of smaller size, such as cellular phones, video cameras, and notebook computers, have been developed at a remarkable pace. In accordance with this trend, there has been increasing demand for compact secondary batteries of high performance. Particularly, a lithium-ion secondary battery is suitably employed as a power source for a variety of portable electronic apparatuses, since it has high energy density and long service life. Therefore, in recent years, production of lithium-ion secondary batteries has drastically increased, and a further increase is expected in the future.
Graphite material is employed in the anode of a lithium-ion secondary battery, and thus in correspondence with an increase in demand for the battery there has been keen demand for graphite powder.
Carbon material that is easily converted to graphite (hereinafter called xe2x80x9ccarbon material of easy graphitizationxe2x80x9d) has become of interest as a material for the aforementioned battery or as a filler for a composite material, and extensive studies have been performed on a variety of such carbon materials.
For example, in order to enhance capacity characteristics of the battery, the crystallinity of graphite must be improved, and thus the carbon material must be subjected to heat treatment at 2,500xc2x0 C. or more for graphitization.
Conventionally, the following two methods are available for producing a mass of graphite micropowder:
(1) a method in which material of easy graphitization is heated at high temperature, and then the graphitized material is crushed, producing graphite powder.
(2) a method in which material of easy graphitization is crushed in advance, and then the crushed material is heated at high temperature.
In method (1), carbon material of easy graphitization, such as any of a variety of cokes, is heated by means of resistance heating of filler carbon powder in powder to which electricity is supplied; i.e., the material is graphitized by means of an Acheson furnace. Alternatively, the material is graphitized by means of a heating furnace comprising a graphite heater. Subsequently, the resultant graphite is crushed, producing graphite powder. At the present time, the method is typically employed, but the method involves some drawbacks. For example, graphitized carbon is smooth (i.e., graphitized carbon is usually employed as a lubricant), and when the graphitized carbon is crushed, flakes tend to form. Thus, when such flakes are employed in an electrode, the flakes are deposited on the surface of the electrode, the surface becomes mirror-like, and the electrode has poor permeability for an electrolyte. As a result the performance of a battery comprising the electrode may deteriorate. Therefore, method (1), in which graphitized material is crushed, cannot produce graphite powder which is suitable for use in batteries and a variety of composite materials; i.e., which exhibits excellent characteristics.
In method (2), material to be treated; i.e., carbon material of easy graphitization, such as coke, is crushed in advance into powder of a suitable particle size. Subsequently, the powder is placed and sealed in a crucible made of carbon, and the crucible is placed in a furnace for graphitization, to thereby produce graphite powder.
This method is preferable from the view that coke is easily crushed as compared with graphite, and that flakes rarely form when coke is crushed as compared with when graphite is crushed.
Therefore, in method (2), graphite powder which is suitably employed in the anode of a lithium-ion secondary battery is produced. However, method (2) involves some problems with regard to heat treatment, as follows.
Carbon material to be heated assumes the form of powder, and thus the material must be placed in a heat-resistant container such as a crucible before the material is heated. Conventionally, a variety of apparatuses and methods have been available for heating the material in a container such as a crucible.
For example, as described above, a crucible containing carbon material is buried in coke powder which is placed in an Acheson furnace, electricity is supplied to the coke powder, and the carbon material in the crucible is heated by means of heat generated from the coke, to thereby graphitize the carbon material. However, this method is of batch-type, and thus a prolonged period of time is involved in carrying out a cycle involving elevating the furnace temperature, heating at a predetermined temperature, and lowering the furnace temperature. In addition, placing the coke powder in the furnace is troublesome, as is removing the powder after completion of heat treatment. Therefore, productivity is considerably poor and the process is unsuitable for mass production.
Furthermore, there is a possibility that the carbon material to be heated will be contaminated with gas of sulfur or metal which is generated from the filler powder and migrates into the carbon material. Such contamination caused by migration of gas may deteriorate the characteristics of graphite carbon powder and may impair battery characteristics.
The temperature within the furnace may vary greatly from position to position in the furnace in accordance with the packing density of the filler powder. Therefore, crucibles containing the carbon material must be placed in the furnace such that the temperatures of the crucibles become as uniform as possible, and thus control of the crucibles may be troublesome. In addition, in order to make the temperatures of the crucibles uniform, the crucibles must be heated for a relatively long time. As a result, carbon material particles tend to stick to one another, and therefore require re-crushing.
A high-frequency induction furnace or a resistance furnace comprising a heater have also been used. These furnaces are provided with a tubular heating zone, and crucibles, whose size is commensurate with the inner diameter of the tube, are continuously passed through the tubular portion in one direction for heating. In such a furnace, gas is not generated, and material can be subjected to heat treatment continuously.
However, in a method in which a furnace comprising a graphite tube serving as a heater is employed, for example, a crucible and powder contained in the crucible are heated by means of heat which is transferred or radiated from the tube. Therefore, in order to raise the temperature of material to be heated to approximately 3,000xc2x0 C., the heater must be heated to a temperature greatly in excess of 3,000xc2x0 C. However, at a temperature higher than 3,000xc2x0 C., the heater itself is considerably consumed, and the service life of the heater is shortened. Incidentally, in order to treat a large amount of the material, the size of the crucible must be increased, and in accordance with an increase in crucible size, the size of the tube must be increased. In addition, the number of heaters must be increased, which causes an increase in equipment costs. Therefore, employing the method industrially is difficult.
There has conventionally been a method in which high-frequency is employed and heating is carried out by means of induction current. The method is efficient from the view that material contained in a crucible is continuously passed through a graphite tube. However, the material assumes the form of powder, and thus resistance of the material is too high for employment of induction heating of the material. Therefore, in order to heat the material, induction heating of the tube or the crucible must be employed.
In the method, heat that is radiated from the tube is mainly employed for heating the material, and when heating is carried out at 3,000xc2x0 C. or higher, the tube itself is considerably consumed and impaired. In addition, graphite tubes are usually expensive. Furthermore, the method requires large-scale apparatuses such as an induction coil and a high-frequency oscillator. In order to treat a large amount of the material, a larger-scale apparatus is required. As a result, the cost of the apparatus increases, and maintenance and control of the apparatus become troublesome.
There is a common problem in relation to heat treatment of carbon powder contained in a crucible. Namely, when any apparatus is employed, the bulk density of carbon powder and the packing ratio of carbon powder are low. Therefore, since electrical conductivity and heat conductivity of the carbon powder are low, neither the size of a crucible nor the size of the apparatus can be increased.
In connection with carrying out graphitization through heating at a high temperature, for example, at 2,500xc2x0 C. or higher, of material of easy graphitization which has been formed into powder or particles in advance, the present invention provides the following:
(1) a method and an apparatus for graphitizing a large amount of the material efficiently within a short period of time at low cost;
(2) prevention of sintering of the material, to the extent possible, through heat treatment within a short period of time;
(3) elimination of contamination by impurity gas during graphitization of the material so as to eliminate the adverse effect of the impurity on characteristics of a battery in which the graphite is employed; and
(4) suppression of operation cost which is attributed to exchange of apparatus parts.
In order to solve the aforementioned problems, the present inventors have performed extensive studies, and have found that excellent graphite powder can be produced by heating a container containing carbon powder, through a supply of electricity to the container.
Accordingly, the present invention provides:
(1) A method for producing graphite carbon powder, characterized by filling a container made of carbon with carbon powder which has been prepared from carbon material through crushing in advance, and heating the carbon powder for graphitization by means of ohmic-resistance heating of the container through a supply of electricity to the container.
(2) A method for producing graphite carbon powder as described in (1) above, wherein the container is employed in a plurality of numbers such that the containers are stacked one on another, electricity is applied from one end of the stacked containers to an opposite end, and ohmic resistance at contact faces of the stacked containers is utilized as a main source of ohmic-resistance heating.
(3) A method for producing graphite carbon powder as described in (1) above, wherein the container is divided into portions in a direction perpendicular to a longitudinal direction of the container, and the divided portions are assembled to constitute a single container.
(4) A method for producing graphite carbon powder as described in any one of (1) to (3) above, wherein electricity is applied from a water-cooled graphite guide electrode which is pressed to the end of the container.
(5) A method for producing graphite carbon powder as described in (4) above, characterized in that graphite material which is inserted between the end of the container and the guide electrode prevents heat loss at the end of the graphite container.
(6) A method for producing graphite carbon powder as described in (5) above, characterized in that the graphite containers, the graphite materials and the guide electrodes are covered with carbon powder and/or carbon fiber so that the heating part is insulated and prevented from oxidatior
(7) A method for producing graphite carbon powder as described in (5) above, wherein at least any one of the part of the graphite containers, the graphite materials or the guide electrodes is placed in an inert gas atmosphere.
(8) A method for producing graphite carbon powder as described in any one of (1) to (3) above, wherein the carbon powder is heated at a temperature of 2,500xc2x0 C. to 3,300xc2x0 C.
(9) A graphite powder which is prepared by means of the method as described in (8) above and which has an interlayer distance (C0) in a C-axis direction in crystal of 6.730 xc3x85 or less.
(10) A graphite powder which is prepared by means of the method as described in (8) above and which has an interlayer distance (C0) in a C-axis direction in crystal of 6.725 xc3x85 or less.
(11) A heating apparatus for graphite carbon powder which comprises a heating chamber including a feeding section for feeding a container filled with carbon powder; a heating section for heating the container, the heating section comprising terminal electrodes for supplying electricity to the container; and a removal section for removing the container after heating, and which apparatus allows the container to be conveyed from the feeding section to the heating section and then to the removal section, so that the container is heated within the heating chamber through a supply of electricity thereto.
(12) A graphite powder which is prepared by means of the method as described in (11) above and which has an interlayer distance (C0) in a C-axis direction in crystal of 6.730 xc3x85 or less.
(13) A graphite powder which is prepared by means of the method as described in (11) above and which has an interlayer distance (C0) in a C-axis. direction in crystal of 6.725 xc3x85 or less.
(14) An electrode material for a lithium-ion secondary battery which makes use of the graphite carbon powder as described in any one of (9),(10),(12) or (13) above.